A large proportion of the "legally blind" population who have useful residual vision and a much larger group of people with reduced visual function, which does not meet the criteria for "legal blindness", have been largely ignored by researchers in vision and in rehabilitation. Our aim is to measure specific vision functions which relate to real-world performance tasks of importance to the visually impaired. Genensky et al (1979) have surveyed the "visual environmental adaptation problems" of the visually impaired; we have used their findings, and those priorities assigned by recent meetings of experts in vision, to decide which functins to examine in our low vision patients. We aim to characterize the visual performance of specific groups of visually impaired patients (senile macular degeneration, cataract, and a heterogeneous group with mobility problems). These patients will have their low contrast acuity, adaptation to changes in environmental light levels, motion sensitivity and sensitivity to glare assessed. These functions will be examined and related to specific performance tests which are based on the underlying vision function. The results of these studies will provide a rational basis for development of new test procedures for the visually impaired.